One consequence of there being such a wide range of communities who have an interest in metadata is that there are a bewildering number of standards and formats in existence or under development. The library world, for example, developed the MARC formats as a means of encoding metadata defined in cataloguing rules and has also defined descriptive standards in the International Standard Bibliographic Description (ISBD) series. More recently, with the advent of complex Web services dealing with diverse types and collections of resources, the number of relevant standards for any given application has mushroomed to the point where it can be difficult to know which is the best or most widely implemented technical standard for any given purpose.
Complexity and Scale of Data
The level of granularity of metadata for any given purpose remains a significant issue, particularly in terms of repositories and similar Web content services that have to reconcile the demands of practical acquisition of content from human or machine metadata creation with the purpose of making content more easily discoverable via complex relationships between resources and entities connected with their creation. Acquisition, especially when human metadata entry is required, demands relatively simple metadata in order to avoid creating a barrier of time and effort that will tend to discourage content depositors. On the other hand, deeper structured metadata can assist in resource discovery and preservation, as well as in providing relationships between resources that can be used as quality linked data for a variety of modern Web and mobile Web services.
Proponents of linked data argue that metadata about both physical and non-physical resources, objects and services are increasingly vital to the provision of services via the Web, particularly on handheld devices, and that such services will soon depend on quality metadata being available as linked data.
Another area in which linked data may be important (and potentially also application profiles, as part of this), is in providing accurate information about the increasingly complex relationships between related resources, for example in knowing which versions, translations or sources a digital resource might have, or equally in the relationships between physical, real-world resources and services and other sources of information about a particular resource or topic.
Large data sets and research data are also an area demanding increasingly complex and subject-specific metadata to be exposed as linked data. How these can be made re-usable by other services and re-used as new resources remains an area of active development. OAI-ORE can be used to describe any kind of resource by providing a wrapper for other resources that can in this way be re-purposed as a new resource, for example in producing a learning object for educational purposes out of a range of pre-existing resources or parts of resources.
From a technical perspective, there are some issues surrounding exactly how metadata will develop in the future. There are proponents of a new but little implemented RDF-based mark-up language for linked data called the Description Set Profile (DSP) language. However, it has not yet been fully demonstrated in practical software solutions how it will bring additional functionality over and above what can be achieved with RDF, or whether such a marginal standard will ever achieve Web scale.
Linked data rely on microformats within Web resources, which are machine-readable but not visible to a human via the browser interface. Whereas it used to be argued by opponents of linked data solutions that a critical mass of Web mark-up, i.e. HTML, XHTML, XML etc would never exist in order to provide a sufficient corpus on which to construct useful Web services, the rise of content management systems and frameworks such as Drupal and Joomla increasingly give the ability to expose their resources as linked data to ordinary users, or indeed perform this seamlessly behind the scenes without their intervention. Consequently, more and more Web tools and services are being coming to depend on the automatic provision and consumption of linked data, particularly on the social Web.